Cathode for cathode ray tube with improved lifetime

ABSTRACT

Impregnated cathode for a vacuum tube comprising an emissive part in the form of a porous pellet impregnated with a compound of alkaline earth metals; the pellet is placed in a dish made of a refractory material and covered with a porous metal foil forming the emissive surface of the cathode. Moreover, the pellet has a separation surface between a heavily impregnated zone and a zone which is not impregnated or weakly impregnated so that the said separation surface comprises at least a hollow part facing the emissive surface. By virtue of the shape of this separation surface, the lifetime of the cathode is improved.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/EP02/23465, filed Nov. 29, 2002, which waspublished in accordance with PCT Article 21(2) on Jun. 19, 2003 inEnglish and which claims the benefit of French patent application No.0115929, filed Dec. 10, 2001.

The subject of the present invention is an impregnated cathode forcathode-ray tubes and more particularly an impregnated cathode with animproved lifetime.

An impregnated cathode consists of a porous metal body, otherwise calleda pellet, which is impregnated by a material called the impregnatingagent capable of emitting electrons, this material being predominantlycomposed of a metal oxide, for example barium oxide. The porous metalbody is generally made of a refractory metal such as, for example,tungsten or molybdenum. The porous metal body is placed inside a metaldish, itself made of a refractory material. A porous metal foil obtainedeither by perforation of a solid foil, or by pressing, sintering andpossible impregnation, is placed above the pellet. The impregnatingmaterial reacts chemically with the pellet and produces the emissivematerial, for example one based on barium or barium oxide, whichemissive material will migrate in the pellet and pass through the poresof the metal foil in order to coat its surface, the said surface thenforming the emissive surface of the cathode.

In the field of cathode-ray tubes, the current trend is towards a greatincrease in the direction of a high increase in the cathode currentdensity in order to use these tubes in high-definition applications, forexample in the field of television. Now, it has been noted that thelifetime of the cathode depended considerably on the current densitythat was required of it.

Various methods have been proposed to increase the lifetime ofimpregnated cathodes, such as for example:

-   -   slower evaporation of the emissive material such as barium,        either by reducing the porosity of the impregnated pellet, or by        lowering the operating temperature,    -   increasing the reservoir of impregnating material and therefore        of emissive material, either by increasing the volume of the        pellet, or by adding a reservoir under the pellet.

However, although these solutions make it possible to increase thelifetime of the cathode at low current density, this lifetime is notsubstantially altered at high current density for the following reasons:

-   -   if the porosity is reduced, this leads to a reduction in the        size of the reservoir of impregnating material    -   if the operating temperature is reduced, the electron emission        decreases    -   the flow of emissive material decreases with the distance which        separates it from the emissive surface, since along the way the        emissive material evaporates in proportion to its surface area        exposed to the vacuum    -   adding a reservoir under the pellet is a particularly expensive        solution which cannot be used for manufacturing mass-market        products and which makes activation of the cathode longer.

The subject of the invention is a particular cathode structure making itpossible to act not on the size of the reservoir of emissive material,but on the flow of emissive material, and more particularly on the rateat which the reservoir is emptied.

For this, an impregnated cathode according to the invention comprises aporous emissive pellet impregnated by an alkaline earth metal compound,the said pellet being placed in a dish made of a refractory material andcovered with a porous metal foil forming the emissive surface of thecathode, characterized in that the pellet has a separation surfacebetween a heavily impregnated zone and a zone which is not impregnatedor weakly impregnated, the said separation surface comprising at leastone hollow part facing the emissive surface.

The invention and its advantages will be better understood using thedescription below and the drawings among which:

FIG. 1 illustrates an embodiment of an impregnated cathode according tothe prior art

FIG. 2 shows a first embodiment of the invention

FIG. 3 illustrates a variant embodiment of a cathode according to theinvention.

An impregnated cathode according to the prior art is described, forexample, in U.S. Pat. No. 4,101,800. A cathode of this sort, illustratedby FIG. 1, comprises a homogeneously impregnated porous pellet 1 made ofemissive materials such as, for example, compounds of alkaline earthmetals such as barium or calcium; the pellet is inserted into a dish 2made of a refractory material such as molybdenum or tantalum. The pelletis covered with a porous metal foil 3 attached to the dish by laserwelding or brazing. The metal foil 3 is, for example, made by pressingand sintering a metal such as tungsten. The dish 2 is secured to ahollow cylindrical sleeve 6 inside which the heating filament 4 of thecathode is placed.

During cathode operation, emissive materials such as barium and bariumoxide are generated in the pores of the pellet and will migrate towardsthe emissive surface and pass through the foil 3 via its orifices 5. Theevaporated barium, diffused by the pellet, continues its path throughthe foil 3, part of which is deposited thereon and another part of whichpasses through in vapour form. The lower the porosity of the foil, thegreater is the part of barium which is deposited thereon. The depositedbarium spreads very quickly and becomes uniform over the surface becauseof the high temperature of the foil, a temperature which is almostidentical to the operating temperature of the pellet. The barium spreadover the upper surface of the perforated foil makes the latter act as anelectron-emissive surface.

The barium flow is caused by a chemical reaction between theimpregnating agent and the material, such as tungsten, constituting thepellet and this flow is directed towards the free path formed by theemissive surface 3. Thus a depletion front 7 is formed, the surface ofwhich is substantially parallel to the emissive surface of the pellet.The depletion front defines a boundary between a zone 8 which is verydepleted in emissive material and which is located directly under thefoil 3 and a deeper zone 9 in which the density of the emissive materialhas not yet been altered. The depletion front will move while having asurface substantially parallel to the emissive surface during the lifeof the cathode, the depth at which the barium is located below theemissive surface increasing progressively with the operating life of thecathode.

The invention is based on the fact that it has been noticed that duringoperation, the depletion front moves while having a surfacesubstantially parallel to its initial surface, like a slow combustionfront. Moreover, the flow of emissive material such as barium decreaseswith the distance separating it from the emissive surface. The result ofthis is that the barium located far from the emissive surface cannot beuseful. As described by A. M. Shroff in “Applications of Surface Science8”, pages 3649, published by North Holland Publishing Company in 1981,the flow of emissive material decreases as the depletion front graduallymoves away from the emissive surface according to a law whereby the flowis inversely proportional to the square root of the time during whichthe cathode has been used.

Two phenomena cause this decrease in flow of emissive material:

-   -   diffusion of the barium oxide towards the surface to be reduced        to barium is increasingly difficult as the residues of chemical        reactions gradually accumulate in the pores.    -   as the depletion front gradually moves away from the emissive        surface, the emissive material evaporates in proportion to its        surface area exposed to the vacuum which is increased in        proportion to the separation.    -   as the depletion front gradually moves away from the emissive        surface, the emissive material has to diffuse along a greater        distance and, on its path, tends to cover a metal surface which        increases with this distance which moreover decreases the amount        of material which reaches the surface.

The invention provides a solution to these problems without altering thechemical phenomena occurring in the pellet, or its porosity, or itsoperating temperature.

As illustrated in FIG. 2, the invention separates the function of theemissive material source and the function of the emissive surface so asto geometrically amplify the flow of emissive material therebetween.

For this, the cathode consists of an emissive pellet 11 inserted in adish 12. The pellet is covered with a porous metal foil 13, for examplemade by pressing and sintering tungsten powder, a metal foil which mayadvantageously be impregnated with impregnating material.

During its manufacture, the pellet 11 has the particular feature ofhaving a heavily impregnated zone 19 and a zone 10 which is weaklyimpregnated or not impregnated at all. The separation surface 18 betweenthese two zones has the geometrical characteristic of being concave andof having at least one hollow between the foil 13 forming the emissivesurface of the cathode and the heavily impregnated part 19 of thepellet.

In the embodiment illustrated in FIG. 2, the separation surface 18 has ahemispherical concavity arranged in the centre of the pellet. The pellet11 has a porosity preferably between 15% and 35%, and a diameter 16 of1.3 mm. Its depth 14 is 0.6 mm and the cavity 10 located at its centrehas a diameter 20 of 0.7 mm. A metal foil 13 is placed over the pellet,which foil may or may not be impregnated, with a porosity between 15%and 35% and having a thickness of 20 μm to 50 μm; the foil is preferablycoated with a layer of alloy such as, for example, osmium/ruthenium orwith iridium which lowers the work function of the electrons. The pelletis inserted into a dish 12 which is secured to a cylindrical sleeve, forexample by laser welding.

The foil 13 and the pellet are secured one to the other by a brazedjoint 21 with a high-melting-point metal base such as molybdenum andruthenium connecting their peripheral surface and surrounding the zone10. The molybdenum/ruthenium based mixture can be prepared from powdersmixed with a solvent, then applied in layers of a few μm in thickness,and finally melted using the laser while pressing the foil and thepreviously impregnated pellet one against the other.

In this way, the depletion front will initially be formed at thehemispherical surface 18, the brazed joint 21 forming a barrierimpermeable to the emissive material coming from the pellet. Thedepletion front will then move while remaining parallel to ahemispherical surface and its surface will increase as the front isgradually forced into the pellet according to a law of the type:d(t)=A·t ^(1/2)

where d is the distance of the front from its initial position, A is acoefficient which depends on the porosity of the pellet and on theoperating temperature, and t is the time.

The surface area of the front will then increase according to a law ofthe type:S(t)=K(R+d(t))² =K(R+A·t ^(1/2))²

where R is the initial radius of the surface 18, and K is a constantcoefficient defined by the portion of sphere represented by the surface18.

Thus the dependence of the flow of emissive material on the time ofpellet use is modified: the increase in the surface area of thedepletion front with the time of cathode use brings an increasinglylarge zone of the impregnated pellet into service, which involves anincreasingly large amount of emissive material; this effectcounterbalances the natural effects of decreasing emissive material flowdescribed above such that, in the structure of the invention, the flowof useful emissive material decreases more slowly than in a structuresuch that of FIG. 1. It will be noted moreover that this advantageincreases with time.

The table below shows the variations in the depletion front, in thebarium flow and in the cathode emission in the case of the prior artillustrated in FIG. 1 and in the case of the invention.

Cumu- lative op- Barium flow erating Depletion (μm) (relative %)Emission (μA) time Standard Inven- Standard Inven- Standard Inven-(weeks) cathode tion cathode tion cathode tion 0 0 0 100 87 6300 6300 524 24 45 44 6300 6300 10 33 33 32 33 6300 6300 15 41 41 26 28 6300 630020 47 47 22 25 6300 6300 30 58 58 18 22 6100 6300 40 67 67 16 20 59006300 50 75 75 14 18 5700 6300 60 82 82 13 17 5500 6300 70 88 88 12 165300 6300 80 94 94 11 16 5100 6300 90 100 100 11 15 4900 6300 100 105105 10 15 4700 6100 110 111 111 10 14 4500 5900 120 115 115 9 14 43005700 130 120 120 9 14 4100 5500 140 125 125 8 14 4000 5300 150 129 129 813 3900 5100

It can be noted that, by setting the lower limit of the cathode emissionto 5100 μA, the lifetime of the cathode went from 80 weeks for thecathode according to the prior art to 150 weeks for a cathode accordingto the invention.

To produce the initial hemispherical surface 18, it is possible to startwith a homogeneously impregnated pellet; after masking part of the uppersurface of the pellet, for example a peripheral zone of the saidsurface, the spatially controlled dissolution of the impregnating agentis carried out so as to create a hemispherical zone with very littleimpregnating agent or none at all.

It is also possible to produce this separation surface 18 from a pellet11 on which a hemispherical zone 10 free of any material is createdmechanically, for example by pressing, then homogeneously impregnated.In this embodiment the emissive material crosses the cavity 10 in vapourform and does not have to cover the surface of the pores as in theprevious embodiment. In this case a problem may be created when thecathode-ray tube equipped with a cathode of this sort is placed undervacuum and particularly when the foil 13 which covers the pellet isporous and impregnated with emissive material; a pressure drop is thencreated between the air contained in the zone 10 and the progressivevacuum prevailing within the tube, a pressure drop which may lead tobreakage of the foil 13. FIG. 3 illustrates an embodiment of theinvention providing a solution to this problem; the cavity 10 is set tothe external pressure by means of at least one channel 30 made in theimpregnated pellet 11 connecting the cavity to the outside of thecathode.

In all cases, in order to obtain an appreciable effect of extending thecathode lifetime, it is desirable that the surface area of the initialseparation surface 18 is greater than the emissive surface of thecathode by at least 20%.

The above embodiments are not limiting; it is advantageously possible toprovide several concave cavities on the surface of the impregnatedpellet or to replace the hemispherical cavity by a surface in the shapeof a half torus.

1. Impregnated cathode, the emissive part of which comprises a porouspellet impregnated by an alkaline earth metal compound, the said pelletbeing placed in a dish made of a refractory material and covered with aporous metal foil forming the emissive surface of the cathode, whereinthe pellet has a separation surface between a heavily impregnated zoneand a zone which is not impregnated or weakly impregnated, the saidseparation surface comprising at least one concave part facing theemissive surface.
 2. Impregnated cathode according to claim 1, whereinthe space located between the concave part and the emissive surface isat least in part, free of any material.
 3. Impregnated cathode accordingto claim 1, wherein the empty space located between the concave part andthe emissive surface communicates with the outer space by virtue of atleast one channel.
 4. Impregnated cathode according to claim 1, whereinthe concave part of the separation surface is obtained by selectivedissolution of the impregnating agent of the pellet.
 5. Impregnatedcathode according to claim 1, wherein the concave part is a portion of aspherical surface.
 6. Impregnated cathode according to claim 1, whereinthe surface area of the concave parts is greater than the emissivesurface by at least 20%.
 7. Impregnated cathode according to claim 1,wherein the surface of the pellet comprises, around the concave part, ametal barrier sealed against diffusion of the emissive material. 8.Cathode according to claim 1, wherein the metal barrier consists of analloy of metals having a high melting point.
 9. Cathode-ray tube whereinit comprises a cathode according to claim 1.